Influenza continues to cause significant morbidity and mortality. In spite of strong recommendations from expert advisory groups, influenza vaccine is not widely used. Reasons for this include (1) difficulty in identifying high-risk patients; (2) uncertainty about the long-term health benefits to patients who are vaccinated. Also, little is known about patterns of use of influenza vaccine. The Manitoba Influenza Study will address each of these questions. This four-year project will use a person-oriented linked record derived from claims data gathered by the Manitoba Health Services Commission (MHSC) for the period 1980-86. The linked record includes demographic data on MHSC enrollees and physicians, influenza vaccinations. Two influenza outbreak periods (1982-83 and 1985-86) will be studied. Five studies will be undertaken: Study 1 will examine the natural history of high-risk conditions to identify factors associated with increased risk of ambulatory care visits, hospitalization, and death due to respiratory conditions during outbreak periods. It will also identify missed opportunities for vaccination. Study 2 will use both retrospective cohort and case-control methods to assess the clinical effectiveness of influenza vaccination in preventing influenza-related ambulatory care visits, hospitalizations, and deaths. Study 3 patient receipt of vaccine. Study 4 will assess known and hitherto unknown adverse effects of influenza vaccination serious enough to require medical care within 3, 7, 14 and 28 days of vaccination. Study 5 will assess the long-term (five-year) outcomes of patients who have had and have not had hospital care for influenza-related illness to determine whether of not "pneumonia is the old man's friend". The Manitoba Influenza Study will provide new information on the clinical effectiveness of influenza vaccination, its safety, and its long-term health benefits. It will characterize patterns of vaccine use and suggest new policies and strategies for influenza vaccination for both the United States and Canada.